Non-chromated aluminum desmutting compositions



Sept. 27, 1966 H. v. SMITH 3,275,562

NON-CHROMATED ALUMINUM DESMUTTING COMPOSITIONS Filed NOV. 12, 1965 Relmionship of Rurio of Maximum Permissible Fluoride Ion Concenrroiion To Ferric Ion Concenirorion us Dependenr on The Rmio of Hydrogen Ion Concenrrorion To Ferric Ion Concenrrorion.

HAROLD VERNON SMiTH BYGM 6-. BMW- A TTORNEY United States Patent 3,275,562 N GN-CHROMATED ALUMINUM DESMUTTING COMPOSITIONS Harold Vernon Smith, Philadelphia, Pa., assignor to Pennsalt Chemicals Corporation, Philadelphia, Pa, a corporation of Pennsylvania Filed Nov. 12, 1963, Ser. No. 322,927 7 Claims. (Cl. 252--147) This invention is directed to compositions for desmutting aluminum and aluminum alloys characterized in that the compositions are free of chromium containing compounds. More particularly, this invention is directed to aqueous solutions for desmutting aluminum and aluminum alloys together with liquid and solid concentrates which are combined with water to form the aqueous desmutting solutions.

Industrial aluminum finishers usually require the alkaline etching of their products to remove surface imperfect-ions and to generally enhance the appearance of the aluminum or aluminum alloys. This alkaline etching leaves on the surface of the aluminum a deposit which, in the trade, is called smut. This smut consists of impurities and alloying elements which are insoluble in the alkaline etching bath. This smut must be removed before any subsequent surface finishing treatment such as brightening or anodizing. If the smut is not removed prior to the surface treatment operation, the surface treatment will be defective and the article will have to be reprocessed.

To remove the smut produced in the alkaline etching of the aluminum or aluminum alloy, desmutting baths are used which are generally aqueous acid solutions. The hydrogen ion can be supplied by sodium bisulfate or sulfuric acid. Since aluminum is attacked by hydrogen ions, proprietary desmutting compositions have heretofore incorporated a hexavalent chromium inhibitor in a water-soluble form such as chromic acid or soluble chromates or dichromates. The hexavalent chromium provides a suitable inhibition of the aluminum and pre- .Vents loss of the metal during the desmutting operation. Recently wildlife conservation and water disposal laws have presented serious problems in regard to the disposal of Wastes containing chromium-laden materials. Many localities now have Water pollution regulations which prohibit or seriously limit the discharge of chromium Wastes into streams. In order to comply with these restrictions aluminum finishers have been required to either neutralize and reduce the chromium wastes in their plants or to have the chromium wastes carried to remote areas for disposal. Either of these courses has greatly increased the cost of using chromium-containing materials for desmutting aluminum.

I have now discovered inexpensive compositions which are non-chromated, which will not corrode aluminum and which have good desmutting properties. In one form of my invention aqueous solutions are employed to desmut the aluminum and aluminum alloys. In another form of my invention, liquid concentrates are prepared Which are diluted in Water to form aqueous acid desmutting solutions. In still another form of my invention, solid concentrates are prepared which are dissolved in water to form the working solutions for desmutting.

The aqueous desmutting solutions all contain hydrogen ion, fluoride ion, and ferric ion, in specified ratios and amounts while being free of substantial amounts of chloride ion. The minimum amount of hydrogen ion which must be present in my working solutions is 018 gram per liter, while the maximum amount of hydrogen ion which may be present is 1.4 grams per liter. The hydrogen ion is the primary ingredient which causes the re- "ice moval of the smut, primarily through the formation of water-soluble salts. The pH of these desmutting solutions are generally Within the range of 0.5 to 1.5. How ever, certain types of smut produced on aluminum and aluminum alloys, particularly alloys which contain substantial amounts of silicon such as 6061, will resist attack by acid and for these types of alloys, fluoride ion must be present in addition to substantial quantities of hydrogen ion.

The fluoride ion must be present to the extent of at least 0.4 gram per liter and must not exceed 3.25 grams per liter. The maximum permissible amount of fluoride in solution is also dependent on the ratio of hydrogen ion to ferric ion as shown by curve AB of the drawing. The ferric ion must be present to the extent of at least 2 grams per liter, while as much as 17 grams per liter may be present in the solutions.

It is also a characteristic of my solutions that they must be substantially free of chloride ions. The presence of chloride ion in acid solutions causes serious attack on aluminum and results in pitting and surface irregularities. By substantial amounts of chloride ion I mean chloride ion in excess of 0.1 gram per liter.

The hydrogen ion for the aqueous solutions can be obtained from sulfuric acid, hydrofluoric acid, sulfamic acid, nitric acid, alkali metal acid salts such as alkali metal acid sulfates and ammonium acid sulfate. Hydrochloric acid is unacceptable as a source of hydrogen ion because of the presence of the chloride ion. Hydrofiuoric acid may be used as a partial source of the hydrogen ion but all of the hydrogen ion could not be derived from this source because of the maximum fluoride ion concentration limit of 3.25 grams per liter, permissible in the aqueous working solutions. If this concentration of fluoride ion is exceeded, then there is no longer suitable inhibition of the attack of hydrogen ion on the aluminum metal. In this circumstance substantial amounts of the aluminum would dissolve, possibly in the form of an aluminum fluoride. The hydrogen ion may also be derived from s-ulfonic and fiuosulfonic acids but these are generally a too costly source of hydrogen ion for industrial applications.

As mentioned previously, my solutions are nonchromated because of the economic and legal problems arising in the disposal of waste materials containing chromium. In the absence of chromium to inhibit the attack of hydrogen ion I have found that ferric ion is an efficient inhibitor. Unexpectedly, the ferric ion assists in the removal of the smut from the aluminum surface, apparently through an oxidation reduction mechanism. My desmutting solutions must contain a minimum of 2 grams per liter of ferric ion, and generally, will not contain in excess of 17 grams per liter. Considerable higher amounts of ferric ion may be used in the solutions, but, ordinarily, no additional benefit is obtained by higher amounts. Moreover, there will be a disadvantageous increase in desmutting time with high ferric content such as 100 grams per liter. Also, a high iron content will decrease the solubilizing ability of the solution for other necessary compounds Which are supplying the hydrogen and fluoride ions.

I have also found that the ratio of hydrogen ion to ferric ion in solution cannot exceed 0.36, if acid attack on the aluminum is to be prevented. This high ratio will give good desmutting in from 0.5 to 2.0 minutes. The practical ratio of hydrogen to ferric ion concentration is 0.03. At this ratio desmutting at F. will take place in about 10.0 minutes whereas if the ratio of hydrogen to ferric ion concentration is as low as 0.006 the desmutting time for aluminum will be about 30.0 minutes and is objectionally high for continuous desmutting operations. Preferably, the ferric ion content will be about 8 grams per liter and the preferred ratio of hydrogen to ferric ion concentration will be at 0.084.

Since the ferric ion is undergoing a reduction to ferrous ion, it will be appreciated that the higher the initial ferric content the longer the desmutting bath can operate Without shutting down to oxidize the ferrous ion to ferric iron. By operating at the preferred iron content and preferred hydrogen to ferric ion concentration ratio, I have been able to operate a desmutting bath continuously for several months before shutting down for restoration of the bath.

The ferric iron may be supplied by one or more compounds selected from the group consisting of ferric sulfate, ferric fluoride, ferric formate, ferric nitrate, and ferric fluosilicate. Ferric fluoride cannot be used as the sole source of ferric io-n because in order to obtain the desired level of ferric iron the maximum permissible content of fluoride for the solution would be exceeded. The same consideration applies to ferric fluosilicate. The preferred source of ferric ion for my solutions is ferric sulfate, because it is inexpensive, relatively soluble in water, and is generally non-hygroscopic.

The fluoride ion must be present in my desmutting solutions in order to facilitate the removal of certain types of smut, particularly from aluminum alloys such as type 6061, which contain considerable silicon. Without fluoride present smut from this type aluminum alloy cannot be removed by iron-inhibited solutions, even at high hydrogen to ferric ion ratios.

As discussed above, the minimum fluoride concentration in my desmutting solutions for eflicient smut removal from all types of alloys is 0.4 gram per liter whereas amounts as high as 3 grarns per liter have been satisfactory.

Since both hydrogen ions and fluoride ions attack aluminum and cause corrosion, it is to be expected that the maximum permissible fluoride, that is the maximum amount of fluoride which can be present Without causing corrosion, will be inversely proportional to the hydrogen ion concentration. Since the ferric ion concentration plays a major role in corrosion inhibition, the relationship between hydrogen ion, ferric ion, and maximum fluoride ion concentration is shown in the drawing.

In the drawing, the ratio of hydrogen ion concentration to ferric ion concentration in aqueous solution is plotted as the abscissa while the ratio of maximum permissible fluoride ion concentration to ferric ion concentration is plotted as the ordinate. The curve AB shows the relationship between the hydrogen ion and fluoride ions in terms of the common ferric ion. Quite unexpectedly, a maximum ratio of 0.23 fluoride ions to ferric ions was discovered at a hydrogen ion to ferric ion ratio of 0.0831. Since the highest possible concentrations of both hydrogen ion and fluoride ion are desired in my desmutting solutions in order to secure the fastest possible desmutting, I prefer to operate my desmutting baths at a fluoride to ferric ion concentration of 0.195 which is near the maximum ratio but still provides a margin of safety against corrosion.

The fluoride ion for my aqueous solutions may be obtained from one or more water-soluble fluorides such as ferric fluoride, ferric flu'osilicate, alkali metal fluorides, alkali metal fluosilicates, alkali metal fluoborates, aluminum fluoride, ammonium fluoride, alkali metal acid fluorides, ammonium acid fluoride and hydrofluoric acid. Preferably, the fluoride is obtained from sodium fluoride because of its low cost and water solubility.

As mentioned above, in the use of my desmutting solutions it is import-ant that no substantial amounts of chloride be present because of the tendency of chloride ions to attack the aluminum metal and cause pitting and other surface irregularities. While not as serious as chloride ion, other ions which should be avoided in my desmutting solutions are bromide, bromate, chlorate, and iodide.

Conveniently, the desmutting of aluminum and aluminum alloy Work pieces is performed at room temperature. At higher temperatures the action is more rapid but there is increased danger of corrosive attack on either the work pieces or on the tanks in which the desmutting baths are contained. Usually, these desmutting baths are contained in stainless steel tanks of such types as 347 and 316. Temperatures as high as F. or higher would cause attack on the stainless steels by these acid solutions.

I have also discovered that it is important in operating my baths that there be a continuous stream of air introduced into the aqueous desmutting solutions. An amount of air should be introduced which produces a gentle agitation of the desmutting solution. This air supplies oxygen which helps to maintain part of the iron in the ferric form. This is a much simpler means of maintaining iron in the ferric form than by chemically treating the solution by adding a liquid or solid oxidizing agent which however, becomes necessary after long periods of bath use.

The work piece is generally immersed for a period of time which may be as little as one minute or as much as fifteen minutes. Generally no benefits are obtained after the latter period. After immersion of the work pieces for the time necessary to remove the smut, the work piece is rinsed with cold water and forwarded to the next finishing operation, which may be brightening or anodizing. The desmutting operation generally follows an etching process which is used for the treatment of all structural and architectural aluminum on which it is desired to give the metal a pleasing appearance.

In addition to the fact that my solutions avoid the chromate disposal problem mentioned above, I have also discovered that my aqueous desmutting compositions can be used at lower concentrations than the comparable chromated desmutting compositions.

Another advantage in the use of my non-chromated solutions for desmutting aluminum is that they may be restored after long use with considerable case. This may be readily accomplished merely by treating the solution with an oxidizing agent, such as hydrogen peroxide, in an amount suflicient to oxidize all ferrous iron to the ferric iron. The extent of oxidation can be followed by a single titration procedure, either determining the ferrous iron content by titration with potassium permanganate or the ferric iron content by titration with stannous chloride. As compared to this simple procedure, a chromated aluminum desmutting solution which has been used so that it no longer properly desmuts must be restored by reacting the solution with fluoride-containing salts in amounts stoichiometrically equivalent to the aluminum ions in solution, three moles of fluoride being added for each mole of dissolved aluminum. This requires that the solution be analyzed for its aluminum content. Moreover, if an excess of fluoride is added to the solution, attack on the aluminum metal will occur. This requires precise chemical treatment for restoring the solution.

Another advantage of my non-chromated desmutting solutions is that the bath life is considerably extended over that of the chromate type of aluminum desmutter. I have observed my desmutting solutions in use continuously for a period of as long as several months without any restoration treatment being necessary. As compared to this, a chromated aluminum desmutter treating the same work pieces has been observed to have a useful bath life of no more than about three weeks.

The concentrates which I supply to the metal finishers can be either liquid or solid compositions. In either case, the compositions are strongly acidic and suitable containers must be provided such as polyethylene-lined steel drums.

In my solid compositions, the hydrogen ion is conveniently supplied in the form of alkali metal acid sulfate, with sodium acid sulfate being preferred over the potas sium salt because of its lower cost. Other sources of ionizable hydrogen for my solid compositions are ammonium acid sulfate and sulfamic acid. My preferred source of ionizable hydrogen is sodium acid sulfate. Either alone or in combination with other sources of erally about 0.09 while the ratio of ionizable fluoride to ferric iron is about 0.2. Other concentrations and ratios of ingredients can be used, of course, so long as the concentrations obtained upon dilution to desmutting concenhydrogen ion sulfuric acid and nitric acid can be adso bed 5 trations are within the limits set forth above for the workon other solid substituents and still give dry solid coming solutions. positions. The ionizable ferric iron which I use in my liquid con- The ferric iron can be suitably supplied from one or centrates can be obtained from ferric sulfate, ferric nitrate, more compounds such as ferric sulfate, ferric fluoride, ferric formate, ferric fluoride, and ferric fluosilicate, or ferric formate, ferric nitrate, and ferric fluosilicate. Be- 10 mixtures thereof. The particular choice of ferric iron will cause of its ready solubility and low cost, ferric sulfat is depend on the cost and solubility in the liquid concentrate. the preferred source of iron. My preferred source of ferric iron are ferric sulfate and The fluoride for the solid compositions can be supplied ferric nitrate. y alkali metal fluoride, ch as sodium and potassium The ionizable fluoride for my liquid concentrates may fluoride, aluminum fluoride, ammonium fluoride, ferric be obtained from one or more compounds which are fluoride, and alkali metal fluosilicates. Generally, the either liquids or which are solids soluble in the aqueous ionizable hydrogen will be present in my solid concenacid concentrate. Suitable compounds are alkali metal mites t0 the eXtent of at least 05% y Weight In addifluorides, aluminum fluoride, ferric fluoride, ammonium tion, the ratio of ionizable hydrogen to ionizable f rr fluoride, ferric fiuosilicate, alkali metal fluoborates, amiI OIl will be about 0.08 while the ionizable fluoride to mon-ium fluoborate, alkali metal acid fiuoridegammonium ferric ion ratio will be about 0.2. acid fluoride and hydrofluoric acid.

It will be appreciated that these solid compositions, as The liquid concentrates are dispersed in water for use well as the liquid concentrates described below, must not in desmutting aluminum and aluminum alloys at a concont ain ionizable chloride in an amount which upon solucentration of between 4 and 32 ounces (avoirdupois) per tion of the concentrate in water at use concentrations, gallon of water. A preferred liquid concentrate for diswill give a chloride content in excess of 0.1 gram per liter. persing in Water to desmut aluminum consists of sulfuric It is also desirable, but not necessary in all cases, to acid (60 B.) 25% by weight, ferric sulfate 21% by have a small amount of a drying agent in my solid comweight, sodium fluoride, 2.5% by weight, and Water positions in order to maintain them free-flowing. M-ag- 51.5% by weight. nesium carbonate is a satisfactory drying agent, although The best method of practicing my invention will be other drying agents such as lithium carbonate, calcium apparent from a consideration of the following examples. silicate, and magnesium silicate can also be used. Two EXAMPLE 1 percent by weight of a drying agent appears to be an adequate amount although higher amounts up to about P l f representative aluminum alloy 2024, 5% can b d 6061 and 60 63 were etched in an aqueous solution con- The solid concentrates are dissolved in water for de- Mining 6 01111665 P gallon Sodium hydroxide at smutting aluminum and aluminum alloys at a minimum f r 10 mi This etching Produced a heavy deposit concentration of four ounces per gallon. Preferably, the f mut n th Surface 0f the aluminum y The addition of the solid concentrate will be about 16 ounces smutted panels were then rinsed in cold water and imper gallon of water although 32 ounces per gallon have mersed in aqueous solutions prepared by dissolving in been used satisfactorily. A preferred form of solid conwater at room temperature .1, 2, 4, l6 and 2 Z centrate consists of sodium acid sulfate 70%, ferric sulfate respectively, of the following solid mixture:

25 sodium fluoride 3%, and magnesium carbonate 2%. Table I In my liquid concentrates the preferred source of oniz- Ingredient: Percent by able hydrogen is sulfuric acid. Another suitable acid for Weight part of the ionizable hydrogen is hydrofluoric acid. Sodium acid sulfate 70 Nitric acid is more corrosive and 15 not as desirable as F ulfate 25 sulfuric acid. If desired, the liquid acids can have solid g? S fl 3 sources of ionizable hydrogen dissolved in them such as 0 Hon 6 sulfamic acid, sodium acid sulfate, and ammonium acid Magnesium carbonate 2 sulfate. It will be appreciated, of course, that any source The solutions formed had the compositions given in Table of hydrogen ion which must be dissolved will decrease 2 below:

Table 2 Concentration of Solid Concentration of Ingredients (grams /liter) Oz./gal. Glramsl NaHSO H Fez(SO Fe NaF F 1 7.5 5. 25 0.044 1.88 0.52 0.23 0.102 2 15 10.5 0. 033 3.75 1.05 0.45 0.204 4 3o 21 0.175 7.5 2. 00 0.9 0.407 s 42 0.35 15 4.18 1.8 0.815 16 120 84 0 30 s. 35 3.0 1. 03 32 240 168 1 40 60 16.7 7. 2 3. 25

the solubilizing capacity of the concentrate for the other ingredients necessary to the composition. The concentration of ionized, or ionizable hydrogen in the liquid concentrate is generally about 0.5% by weight. The

During immersion in the solutions of Table 2 the aluminum alloy panels were watched to see if there were any indications of attack on the metal as indicated by the evolution of hydrogen gas. These observations are noted in ratio of ionizable hydrogen to ionizable ferric iron is gen- Table 3.

From Table 3 above it will be seen that concentrations of at least four ounces per gallon are required before there is complete inhibition of acid attack on the panels.

After immersion for ten minutes the panels were removed from the solutions and the percent smut removed was observed as noted in Table 4.

Table 4 Concentrate Smut Removed, Percent of Complete emoval Oz./gal. pH at 25 C. 1100 2024 6061 6063 alloy alloy alloy alloy These observations point out that at least four ounces per gallon of the concentrate are required for complete removal of smut from all of the alloys tested. However, in order to secure complete smut removal from all alloys in the minimum possible time practical for continuous commercial operations, that is, complete desmutting within two to ten minutes, it was found that a concentration of sixteen to thirty-two ounces of the solid concentrate per gallon of water was required.

EXAMPLE 2 The following solid concentrates were formulated by blending together the various ingredients indicated.

Percent by Ingredient: weight A KI-ISO; 72.5

F62(SO4)3 NaF 2.8

MgCO 1.8

H NSO OH 65.7

FE2(SO4)3 NaF 3.4

MgCO 2.3

Sulfuric acid (98%) 49.3

Fe (SO 42.3

NaF 5.1

MgCO 3.3

NaHSO 57.5

FG(NO3)3'9H2O NaF 2.6

MgCO 1.6

NaHSO 50.5

Ferric ammonium citrate 45.8

NaF 2.2

MgCO 1.5

' noted after the long production run.

Nanso 70 F2(SO4)3 NaBF 3 M co 2 NaHSO 70 F2(SO4)3 N32811 3 M co 2 NaHso 70 F2(SO4 3 NH4F 3 M co 2 Panels of aluminum alloys 1100, 2024, 6061 and 6063 were etched in a 6-oz./gal. solution of 98% sodium hydroxide at 150 F. for 10 minutes to produce a thick layer of smut on the panel surfaces. These smutted panels were carefully rinsed in cold water and immersed in solutions of solid concentrates A-H at room temperature for 10 minutes. The amount of desmutting and degree of attack, if any, are noted in Table 5 for each solution. Complete desmutting of all of the alloy panels, without attack on the alloys was obtained for each solution.

Table 5 Concentrate Grams/liter Percent Smut Corrosion Removed of Panel 131 100 None 100 None 71 100 None 146 100 N one 166 100 None 100 None 120 100 None 120 100 None EXAMPLE 3 Extruded sections of aluminum alloy 6063 and 6351 were first cleaned in an alkaline cleaner containing 70% borax, 17% tetrasodium pyrophosphate, 1% sodium gluconate, and balance wetting agent, at a concentration of 6 ounces per gallon at a temperature of 140 F. for 8 minutes. Following the alkaline cleaning the extruded sections were rinsed in cold water and then etched in a solution of sodium hydroxide containing 98% caustic soda at a concentration of 8 ounces per gallon for a period of 8 minutes at a temperature of F. Following etching, the Work pieces were rinsed in cold water and then placed in a desmutting bath prepared by dissolving in Water a granular concentrate containing sodium acid sulfate 70% by weight; ferric sulfate 25% by weight; sodium fluoride 3% by weight and magnesium carbonate 2% by weight, under conditions of mild agitation.

The granular solid was used at a concentration of 16 ounces per gallon and the solution was held at a temperature of 75 F. The desmutting solution was formulated by dissolving 7000 pounds of dry powder concentrate in 7000 gallons of water. The work pieces were immersed in the solution for a period of five minutes, removed, rinsed with cold water, and thereafter anodized.

After solution was effected and while the desmutting operations were in progress, the bath was continuously agitated by blowing air through the desmutting solution. This desmutting bath was operated continuously and effectively for a period of several months without requiring restoration of the solution. The tank containing the desmutting solution was fabricated from 316-type stainless steel and no evidence of attack on the tank was Complete removal of smut was observed on all of the extruded sections and no visible attack on the aluminum alloys was noted at any time.

EXAMPLE 4 The minimum ferric iron content required to inhibit the attack of hydrogen ion on aluminum alloys 2024 and 6063 was determined by placing cleaned panels of these alloys in aqueous solutions at room temperature containing varying amounts of sodium acid sulfate and varying quantities of ferric sulfate. Satisfactory inhibition was observed when no attack on the immersed panels was noted. Attack on the alloys was evident by either 10 ion concentrations as a function of the hydrogen to ferric ion concentration ratio is plotted as curve AB of the drawing.

Table 8 PERMISSIBLE FLUORIDE CONTENT OF SOLUTIONS Grams per liter Ratio NaHSO; H+ FG2(SO4)3 Fe+++ NaF F- H+/Fe+++ F/Fe+++ 110.8 .926 9.2 2. 57 .96 0. 435 0. 359 0.170 109. 1 .910 10. 9 3. 04 1. 2 0. 543 0. 299 0. 178 102. s 856 17. 2 4. so 2. 04 0. 922 0. 177 0. 192 97. 0 .807 23. 0 6. 42 3. 0 1. 36 0. 126 0. 212 88. 4 736 31. 6 8. s3 4. 44 2. 02 0. 0831 0. 229 78. 0 .65 42. 0 11.72 5. 2. 42 0. 0555 0. 205 72. 0 .60 4s 0 13.40 4. 82 2.185 0. 0447 0.163 60. 0 .50 50. 0 15. 7 4. 92 2. 23 0. 030 0. 134 40. 0 .333 80. 0 22. 3 5. 64 2.55 0. 015 0. 115 20.0 .167 100. 0 27. 9 6. 84 a. 10 0. 006 0.110

observing gas evolution at the interface of the immersed 30 I claim:

panel or by noting decreased reflectivity of the panels after the immersion period. Observations with alloy 2024 are noted in Table 6 and for alloy 6063 in Table 7.

1. Acidic aqueous solution for desmutting aluminum and aluminum alloys consisting essentially of water and hydrogen, ferric and fluoride ions, in which the hydrogen Table 6 ATTACK ON 2024 ALUMINUM SHEET FenSOm Concentration, 0z./gal. NaHSO4 c0110., ozJgal.

Consid Slight Consid Consid Consid Consid Consid Consid Consid Consid Consid.

Table 7 ATTACK ON 6063 ALUMINUM EXTRUSION Fe2(SO Concentration, 0z./gal. NaHSOt c0110., oz./gal.

1.0 Consid None.. None None None None None None None. 4. Consid Consid do do .41 d Do. 8. Consid Consid Sligl do "do Do. 1 Consid Consid do Tra do Do. 3. Consid Consid Cousi Oonsi 'Ira D0.

Tables 6 and 7 show that at least one part of ferric sulfate is required for each twelve parts of sodium :acid sulfate. This gives a hydrogen ion to ferric ion ratio of 0.359.

EXAMPLE 5 The maximum permissible fluoride concentration in solutions which contain suflicient hydrogen to permit rapid desmutting and suflicient ferric ion to inhibit hydrogen ion corrosion was determined by adding sodium fluoride to various desmutting solutions in small increments until sufficient fluoride was present to cause fluoride type of corrosion on test panels immersed in the desmutting solutions. The observations are recorded in Table 8. In Table 8, below, the fluoride concentration in grams per liter is the maximum fluoride content after which further addition of fluoride caused fluoride attack on the alloy for each ratio of hydrogen ion concentration to ferric ion concentration tested.

ion concentration lies within the range of 0.18 to 1.4 grams per liter, in which the ferric ion concentration lies within the range of 2 to 17 grams per liter, in which the fluoride ion concentration lies within the range of 0.4 to 3.3 grams per liter, in which solution the ratio of the hydrogen ion concentration to the ferric ion concentration lies within the range of 0.03 and 0.36 and in which solution the ratio of the fluoride ion concentration to ferric ion concentration does not exceed the values given by curve AB of the drawing for the corresponding ratios of hydrogen ion concentration to ferric ion concentration, the said solution also being substantially free of chloride ion, said hydrogen ion being proat least one compound selected from the group consisting of ferric sulfate, ferric fluoride, ferric formate, ferric nitrate and ferric fluosilicate, said fluoride ion being provided by at least one compound selected from the group consisting of ferric fluoride, ferric fluosilicate, alkali metal fluorides, alkali metal fluosilicates, alkali metal fluoborates, aluminum fluoride, ammonium fluoride, alkali metal acid fluorides, ammonium acid fluoride and hydrofluoric acid.

2. Acidic aqueous solution for desmutting aluminum and aluminum alloys consisting essentially of water and hydrogen, ferric and fluoride ions, in which the hydrogen ion concentration lies within the range of 0.18 to 1.4 grams per liter, in which the ferric ion concentration lies Within the range of 2 to 17 grams per liter, in which the fluoride ion concentration lies within the range of 0.4 to 3.3 grams per liter, in which solution the ratio of the hydrogen ion concentration to the ferric ion concentration lies within the range of 0.03 and 0.36, in which solution the ratio of the fluoride ion concentration to ferric ion concentration does not exceed 0.2, and said solution also being substantially free of chloride ion, said hydrogen ion being provided by at least one compound selected from the group consisting of sulfuric acid, hydrofluoric acid, sulfamic acid, nitric acid, alkali metal acid sulfate, and ammonium acid sulfate, said ferric ion being provided by at least one compound selected from the group consisting of ferric sulfate, ferric fluoride, ferric formate, ferric nitrate and ferric fluosilicate, said fluoride ion being provided by at least one compound selected from the group consisting of ferric fluoride, ferric fluosilicate, alkali metal fluorides, alkali metal fluosilicates, alkali metal fluoborates, aluminum fluoride, ammonium fluoride, alkali metal acid fluorides, ammonium acid fluoride and hydrofluoric acid.

3. Acidic aqueous solution for desmutting aluminum and aluminum alloys consisting essentially of water and 0.7 gram per liter of hydrogen ion, 8.37 grams per liter of ferric ion, and 1.6 grams per liter of fluoride ion, the said solution also being substantially free of chloride ion, said hydrogen ion being provided by at least one compound selected from the group consisting of sulfuric acid, hydrofluoric acid, sulfamic acid, nitric acid, alkali metal acid sulfate, and ammonium acid sulfate, said ferric ion being provided by at least one compound selected from the group consisting of ferric sulfate, ferric fluoride, ferric formate, feric nitrate and ferric fluosilicate, said fluoride ion being provided by at least one compound selected from the group consisting of ferric fluoride, ferric fluo silicate, alkali metal fluorides, alkali metal fluosilicates, alkali metal fluoborates, aluminum fluoride, ammonium fluoride, alkali metal acid fluorides, ammonium acid fluoride and hydrofluoric acid.

4. An acidic aqueous solution for desmutting aluminum and aluminum alloys consisting essentially of water and about 84 grams per liter of sodium acid sulfate, 30 grams per liter of ferric sulfate, 3.6 grams per liter of sodium fluoride and 2.4 grams per liter of magnesium carbonate, the said solution also being substantially free of chloride ion.

5. A solid composition for dissolving in water to form an acidic solution for desmutting aluminum and aluminium alloys consisting of by Weight sodium bisulfate, 25% by weight ferric sulfate, 3% by weight sodium fluoride and 2% by weight of a drying agent selected from the group consisting of magnesium carbonate, lithium carbonate, calcium silicate and magnesium silicate, the said solid composition being substantially free of chloride.

6. The composition of claim 5 in which the drying agent is magnesium carbonate.

7. A solid composition for dissolving in water for desmutting aluminum and aluminum alloys, said composition consisting essentially of (a) at least one hydrogen ion providing compound selected from the group consisting of sulfuric acid, hydrofluoric acid, sulfamic acid, nitric acid, alkali metal acid sulfate, and ammonium acid sulfate, in an amount sufficient to provide from about 0.18 gram to about 1.4 grams of hydrogen ion per liter of working solution,

(b) at least one ferric ion providing compound selected from the group consisting of ferric sulfate, ferric fluoride, ferric formate, ferric nitrate, and ferric fluosilicate in an amount suflicient to provide from about 2 grams to about 17 grams of ferric ion per liter of working solution, and

(c) at least one fluoride ion providing compound selected from the group consisting of ferric fluoride, ferric fluosilicate, alkali metal fluorides, alkali metal fluosilicates, alkali metal fluoborates, aluminum fluoride, ammonium fluoride, alkali metal acid fluorides, ammonium acid fluoride and hydrofluoric acid in an amount suflicient to provide from about 0.4 gram to about 3.3 grams of fluoride ion per liter of working solution,

said solid composition also being substantially free of chloride and the proportions of said compounds being such that when the solid composition is dissolved in suflicient water to form a Working solution the ratio of the hydrogen ion concentration to the ferric ion concentration in the solution lies within the range of 0.03 and 0.36 and the ratio of the fluoride ion concentration to the ferric ion concentration does not exceed 0.2.

References Cited by the Examiner UNITED STATES PATENTS 70,705 11/1867 Du Motay et al 252186 1,599,996 9/ 1926 Fritz 242142 X 1,765,344 6/1930 Pietzsch et al. 252186 2,316,219 4/1943 Brown et al 252142 X 2,474,526 6/1949 Healey et al. 252142 2,564,549 8/1951 Stargardter 252--142 X 2,828,193 3/1958 Newman 25279.3 X

LEON D. ROSDOL, Primary Examiner.

JULIUS GREENWALD, ALBERT T. MEYERS,

Examiners.

W. E. SCHULZ, Assistant Examiner. 

1. ACIDIC AQUEOUS SOLUTION FOR DESMUTTING ALUMINUM AND ALUMINUM ALLOYS CONSISTING ESSENTIALLY OF WATER AND HYDROGEN, FERRIC AND FLUORIDE IONS, IN WHICH THE HYDROGEN ION CONCENTRATION LIES WITHIN THE RANGE OF 0.18 TO 1.4 GRAMS PER LITER, IN WHICH THE FERRIC ION CONCENTRATION LIES WITHIN THE RANGE OF 2 TO 17 GRAMS PER LITER, IN WHICH THE FLUORIDE ION CONCENTRATION LIES WITHIN THE RANGE OF 0.4 TO 3.3 GRAMS PER LITER, IN WHICH SOLUTION THE RATIO OF THE HYDROGEN ION CONCENTRATION TO THE FERRIC ION CONCENTRATION LIES WITHIN THE RANGE OF 0.03 AND 0.36 AND IN WHICH SOLUTION THE RATIO OF THE FLUORIDE ION CONCENTRATION TO FERRIC ION CONCENTRATION DOES NOT EXCEED THE VALUES GIVEN BY CURVE AB OF THE DRAWING FOR THE CORRESPONDING RATIOS OF HYDROGEN ION CONCENTRATION TO FERRIC ION CONCENTRATION, THE SAID SOLUTION ALSO BEING SUBSTANTIALLY FREE OF CHLORIDE ION, SAID HYDROGEN ION BEING PROVIDED BY AT LEAST ONE COMPOUND SELECTED FROM THE GROUP CONSISTING OF SULFURIC ACID, HYDROFLUORIC ACID, SULFAMIC ACID, NITRIC ACID, ALKALI METAL ACID SULFATE, AND AMMONIUM ACID SOLFATE, SAID FERRIC ION BEING PROVIDED BY AT LEAST ONE COMPOUND SELECTED FROM THE GROUP CONSISTING OF FERRIC SULFATE, FERRIC FLUORIDE, FERRIC FORMATE, FERRIC NITRATE AND FERRIC FLUOSILICATE, SAID FLUORIDE ION BEING PROVIDED BY AT LEAST ONE COMPOUND SELECTED FROM THE GROUP CONSISTING OF FERRIC FLUORIDE, FERRIC FLUOSILICATE, ALKALI METAL FLUORIDE, ALKALI METAL FLUOSILICATE, ALKALI METAL FLUOBORATES, ALUMINUM FLUORIDE, AMMONIUM FLUORIDE, ALKALI METAL ACID FLUORIDES, AMMONIUM AICD FLUORIDE AND HYDROFLUORIC ACID. 